The present invention relates to rotary compressors or blowers, particularly to blowers of the Roots type. More particularly, the present invention relates to blowers of the type having rotors press-fit on shafts for rotation therewith.
Although the present invention may be used with various types of blowers, it is especially advantageous when used with a Roots type blower, and will be described in connection therewith.
Rotary blowers of the Roots type typically include a pair of meshed, lobed rotors, with each of the rotors being mounted on a shaft, and each shaft having mounted thereon a timing gear. Rotary blowers, particularly Roots-type blowers, which are employed as superchargers for internal combustion engines normally operate at relatively high speeds, typically in the range of 10,000 to 20,000 rpm.
As is well known to those skilled in the art, it is essential that the rotors mesh with each other, to transfer volumes of air from the inlet port to the outlet port, without the rotors actually touching each other. Therefore, the connection between each rotor and its shaft must be such that there is no relative rotation possible between the rotor and its shaft. In addition, transferring volumes of air at greater than atmospheric pressure requires that the mounting of the rotor on its shaft be able to transmit a certain amount of torque.
A typical prior art arrangement for mounting an aluminum rotor on a steel shaft is illustrated and described in U.S. Pat. No. 4,595,349, assigned to the assignee of the present invention, and incorporated herein by reference. In the cited patent, the steel shaft includes two different press-fit diameters, as well as a set of external splines which, during the press-fit operation, deform the mating surface of the bore of the rotor, thus providing a torque-transmitting connection between the shaft and the rotor.
It has been found that, because of the different coefficients of thermal expansion of steel and aluminum, it is necessary to provide a greater amount of interference between the shaft and the bore surface before the press-fit is performed. The result is that there is still a sufficient interference fit while the supercharger is operating at temperatures much higher than those at which the press-fit was performed. However, a problem associated with the greater amount of interference, and the heavier press-fit required, is a certain amount of "shearing-off" of aluminum from the rotor bore surface during the press-fit, the aluminum being sheared off in the form of either rings or chips. Those that are loose do not present a serious problem, but those that cannot be removed by conventional deburring methods are very undesirable. Complete removal of such material would require expensive, extra processing, but the failure to completely remove such material could potentially result in serious damage to the supercharger, if the material subsequently becomes loose during operation.